Method for improving the storage stability of ammonium salts



the surrounding atmosphere.

United States METHOD FOR IMPROVING THE STORAGE STABILITY OF AMMONIUM SALTS Eugene D. Guth, Bartlesville, kla., assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Filed May 13, 1955, Ser. No. 508,294

17 Claims. (Cl. 71-64) This invention relates to a storage stable ammonium salt, such as an ammonium nitrate. In one aspect it relates to a method for improving the storage stability of ammonium salts, such as ammonium nitrate, by incorporating a novel stabilizing agent therewith. In one of its more specific aspects it relates to fertilizers containing ammonium salts, such as ammonium nitrate, and a novel stabilizing agent. In a further specific aspect, it relates 'to an improved rocket propellant.

Ammonium nitrate has enjoyed wide use of a fertilizer because of its high nitrogen content and the ready availability of the nitrogen. Like salt-peter and other nitrates,

. ammonium nitrate readily and easily forms explosive mixtures with other combustible substances. One of the difliculties experienced in its use, however, is its tendency to set, or cake, under conditions of shipping or storage. Another difliculty is that due to the thermal decomposi tion at elevated temperatures and pressures.

It is generally believed that caking occurs during storage when ammonium nitrate changes in crystal habit from plates or dendrites to needles as a result of solution and recrystallization at the surface of the ammonium nitrate particles brought about by the absorption of moisture from In the solid state, ammonium nitrate occurs in five difierent crystal modifications with definite transition temperatures and when the salt is caused to pass from one crystal phase to another,

this contributes to the caking tendency, although it is I parafiin, kaolin, kieselguhr, plaster of Paris, soapstone,

and the like. The use of certain inorganic salts, e.g., magnesium nitrate and calcium nitrate, have also been employed. Their application usually involves coating or dusting methods.

Ammonium nitrate is deliquescent and it takes up water from the atmosphere readily. This property complicates its use as a fertilizer and explosive. Though generally ammonium nitrate is a stable salt, careful handling is required and the complications resulting from its deliquesence may be overcome in most instances by such handling and the coating of the crystals with certain inert mineral and organic materials. While some of these materials are satisfactory in lessening the tendency for the salt to cake, their application usually involves costly processes and may result in unduly thick coating of some crystals and a discontinuous coating of other crystals. Organic materials are, in addition, objectionable because they assist the thermal decomposition of ammonium atent nitrate at elevated temperatures and pressures and in some instances have even proved hazardous.

Particular attention has been focused on the thermal decomopsition of ammonium nitrate in addition to its caking tendency. Such attention in the main has been caused by several catastrophes resulting from the thermal decomposition of ammonium nitrate with explosive violence (e.g., Oppau, Germany, in 1921, and Texas City in the United States in 1947). These castastrophes indicate that ammonium nitrate is, under certain conditions of temperature and pressure, explosive per se. Ammonium nitrate decomposes readily when heated and at high temperatures and pressures it detonates. The ammonium nitrate involved in the Texas City disaster was coated with certain oxidizable material to prevent .caking. This coatlng material it is believed added to the hazard created by the elevated temperatures and pressures resulting from .a fire occurring on board the ship which was transporting the ammonium nitrate.

This invention involves the incorporation of a nonoxidizable stabilizing agent in an ammonium salt, such as solid ammonium nitrate, whereby the storage stability of the salt is improved, tending to make it less susceptible to caking and inhibiting its thermal decomposition. The stabilizing agent which I preferably employ is magnesium nitride in an amount suflicient to react with the salt to liberate ammonia which inhibits the thermal decomposition of the salt and also lessens its tendency to cake. I can also use metal amides as the stabilizing agent.

Accordingly, it is an object of this invention to provide a storage stable ammonium salt, such as ammonium nitrate, composition.

Another object is to provide a method to lessen the tendency of an ammonium salt, such as ammonitun nitrate, to cake upon storage.

A further object is to provide a method to inhibit the thermal decomposition of ammonium salts, such as ammonium nitrate.

A further object is to provide an improved fertilizer.

A still further object is to provide an improved rocket propellant. 1

Other objects and advantages of this invention will become apparent to one skilled in the art upon reading the following disclosure and discussion.

I have now discovered that certain stabilizing agents may be incorporated in ammonium salts, such as ammonium nitrate, which will not only lessen the tendency of the salt to cake but also will inhibit the thermal decomposition of the salt. The stabilizing agents useful in the practice of this invention include certain metal nitrides. While certain metal amides are useful in the practice of this invention, generally they are extremely reactive and must be stored under inert liquid, thus making them less preferable. While any metal nitride is useful in the practice of this invention, the nitrides of certain metals of groups IA and IIA of theperiodic table are preferred. These include lithium, sodium, potassium, magnesium, calcium, and barium. Aluminum nitride is also applicable in the practice of this invention. These nitrides may be prepared either by direct union of the elements or by deammonation of the amides at elevated temperatures.

On being heated, ammonium nitrate can undergo decomposition in a number of ways, depending upon the conditions of heating. Decomposition does not appear to begin until the salt melts (approximately C.). When the salt is heated beyond its melting point (around 170 C.) up to about 230 C., it decomposes chiefly according to the equation:

NH NO N O+2H O The dissociation ofammonium nitrate also forms s'ome ammonia and nitric acid. When confined in a tube and heated the liberation of an excessof ammonia completely stops the decomposition, even at 5060 C. above the normal point of decomposition. If the salt is confined :and isuperheated from .230 C. upward, the decomposition becomes :morezrapid:andsendsbybecoming explosive at fthe same :time .the salt becomes incandescent. There TareJ-atleast seven ways .the'salt can 'decompose,:some of .these reactions :proceed as concurrent or side reactions with the formation of varying amounts of hi'gheroxides -of nitrogen.

I have found that when magnesium nitride is mixed with ammonium nitrate the odor of ammonia -is readily observed even "at room temperature. The reaction can be observed by:noting apressure increase -in theabsence of any water-or air. When-asu'flicient amount-of magnesium nitride is mixed with the ammonium nitratesalt, the tendency for the salt to-cake during'storage is'sub- "stantially lessened.

The inhibition of the thermal decomposition of ammonium nitrate by the incorporation of a sufficient amountofmagnesium nitride is illustrated by-the following example.

A dried sample of ammonium nitrate weighing 0.300 gram was placed in a closedreaction tubein asystem provided with a pressure gage and a manifold tube for adding and removing gases. The sample sizewas calculated to give about'ZO p.s.i. absolute on total decomposition and the pressure was registered by a Wallace- 'Tierman pressure gage. The remainder of the system consisted of a tube with several side arms, each with a stopcock, for evacuating the system, admitting nitrogen, and admitting other desired gases. 'The'reaction tube was enclosed by a furnace which was preheated several degrees 'above the reaction temperature. The reaction tube Was then evacuated to insure against the presence of substantial amounts of moisture "being present. The sample was'heated rapidly "to within a few degrees of .the desired temperature and final temperature adjustments were made as quickly as. possible by adjustment with a Variac. The temperature was quickly brought up to 210 C. and the pressure reading'was recorded during intervals of time. Since the decomposition of the ammonium nitrate is dependent upon the time of heating, the measurement of the increase in pressure generated'by the decomposition gases as a function of time can be translated into a decomposition rate. A dried sample of ammonium nitrate weighing 0300 gram and containing 2 percent by Weight of magnesium nitride was also decomposed according to the same procedure outlined above. 'The results of these decompositions are .set forth inTablelI.

TABLE 1 Thermal decomposition of ammonium nitrate and ammonium nitrate'with magnesium nitride :Itis apparent-fromthe-results set forth in .TableI that i the decomposition of ammonium nitrate is inhibited when a small amount of magnesium nitride is incorporated therewith.

I have found that upon heating a confined sample of ammonium nitrate admixed with a small amount of magnesium nitride a slight pressure is observed even at low temperatures (i.e., below 170 .C.). The slight pressure remains constant after the initial build-up. At 170 C. the-reaction-is s'low dueto'thepresence of the ammonia generated. This ammoniacaused slower decomposition of 'the sample and when the ammonia was pumped off the decomposition proceeded at-a'fairlyhigh rate. However, during a long period of time the pressure did not build upappreciablyafter'once buildingnp to a maximum pressure.

A mixture containing equal proportions of ammonium nitrate and magnesium nitride was heated to 100 C. and the temperature maintained at this point for one hour during which a continuous pressure was noted. Thus, it'is apparent that even at temperatures lower than the melting point of ammonium nitrate, the magnesium nitride of the present invention will react to produce the gases which inhibit rapid decomposition.

lt has beenfound that-when samples of ammonium nitrate admixed with a small amount ofmagnesium nitride, in accordance with the practice of this invention, were left exposed to ambient temperatures and humidity, no 'caking tendency of the samples occurred, while samples of ammonium-nitrate alone caked under the sameconditions.

Generally-the amount of magnesium nitride to be employed in the practice of this invention is in the range of 0:5 to 5 percent by weight of the ammonium salt, such as ammonium nitrate, and preferably in the range of 1.0 to 2.0 percent. Obviously in the caseof a fertilizer, the amount of magnesium nitride employed should not be so excessive that'the nitrogen content of fertilizer is reduced by a substantial amount. The most feasible amount to be employed will be apparent to those skilled in the art upon consideration of the application to which the ammonium nitrate is put, and other considerations. Thus, it is not intended herein that any fixed rule be set forth in that regard.

Ammonium nitrate compositions prepared by thepractice of this invention finds particular application in rocket propellants. Magnesium nitride is preferably employed as the stabilizing agent in the above stated amounts and -.can be readily compounded with binder compositions,

such as rubber-like materials, asphalt, etc. A binder such as a copolymer of methylvinylpyridine and butadiene is particularly applicable. Propellants so prepared generally will .have 10 to 20 parts by weight of binder and 90 to parts by weight of the stabilized ammonium nitrate.

While the practice of this invention 'has been found particularly advantageous in lessening the cak-ing tendency of ammonium nitrate, it can be applied-to certain other ammonium compounds, "especially those suitable as fertilizers, such as-ammonium sulfate, ammonium carbonate, ammonium phosphate, and the like, which likewise exhibit a tendency to cake upon storage. The fertilizer produced by the practice of this invention maybe mixed with other'materials known inthe art.

While theinvention has been described particularly in connection with ammonium nitrate and. magnesium nitride, and other preferred .embodiments of this invention, it

is to be understood that this description is illustrative only and is not intended to limit the invention. Various modifications will be apparent to one skilled in the art upon study of this disclosure and it is believed such modifications are within the spirit and the scope of this invention.

Havingdescr-ibedmy invention, I claim:

1. A method for improving the storage stability of an ammonium salt, which comprises incorporating in said salt .a small but stabilizing amount of a metal nitride.

2. A method for improving the storage stability of an ammonium salt selected irom. the group consisting of ammonium nitrate, ammonium sulfate, ammonium carbonate and ammonium phosphate, which comprises incorporating in said salt a small but stabilizing amount of a metal nitride.

3. The method according to claim 2 wherein the amount of said metal nitride is in the range of 0.5 to 5.0 percent by weight of the ammonium salt.

4. The method according to claim 2 wherein the amount of said metal nitride is in the range of 1.0 to 2.0 percent by weight of the ammonium salt.

5. The method of claim 2 wherein said metal nitride is magnesium nitride.

6. The method of claim 2 wherein said metal nitride is calcium nitride.

7. The method of claim 2 wherein said metal nitride is aluminum nitride.

8. A method for improving the storage stability of ammonium nitrate, which comprises mixing said salt with magnesium nitride in an amount in the range of 0.5 to 5.0 percent by weight of the ammonium salt.

9. A new composition of matter comprising a solid ammonium salt and a minor but storage stabilizing amount of a metal nitride.

10. A new composition of matter comprising an ammonium salt fertilizer selected from the group consisting of ammonium nitrate, ammonium sulfate, ammonium carbonate and ammonium phosphate, and a minor but storage stabilizing amount of a metal nitride.

11. A new composition of matter according to claim in which said salt is ammonium nitrate.

12. A fertilizer according to claim 10 in which said metal nitride is present in an amount in the range of 0.5 to 5.0 percent by Weight of the ammonium salt.

13. A fertilizer according to claim 10 in which said metal nitride is present in an amount in the range of 1.0 to 2.0 percent by weight of the ammonium salt.

14. A fertilizer according to claim 10 wherein said metal nitride is magnesium nitride.

15. A fertilizer according to claim 10 wherein said metal nitride is calcium nitride.

16. A fertilizer according to claim 10 wherein said metal nitride is aluminum nitride.

17. A new composition of matter comprising solid ammonium nitrate in admixture with magnesium nitride in an amount in the range of 0.5 to 5.0 percent by weight of the ammonium salt.

References Cited in the file or this patent UNITED STATES PATENTS 1,042,723 Sinding-Larsen Oct. 29, 1912 1,916,841 Klippel et -al. July 4, 1933 2,399,987 Cordie et al. May 7, 1946 2,417,115 Leon et al. Mar. 11, 1947 2,434,872 Taylor et al. Ian. 20, 1948 2,455,205 Whetstone et a1 Nov. 30, 1948 2,616,786 Whetstone Nov. 4, 1952 OTHER REFERENCES Industrial and Eng. Chem, Production of Grained Ammonium Nitrate Fertilizer, Miller et el., vol. 38, No. 7, pages 709-718, July 1946. 

1. A METHOD FOR IMPROVING THE STROAGE STABILITY OF AN AMMONIUM SALT, WHICH COMPRISES INCORPORATING IN SAID SALT A SMALL BUT STABILIZING AMOUNT OF A METAL NITRIDE. 